Method of altering the visible absorption characteristics of permanently tinted photochromic glasses

ABSTRACT

A photochromic glass having colorant incorporated in the glass composition to impart a permanent tint, and having its chromaticity point shifted from the blue-green side toward the red side of the white point in the CIE chromaticity coordinate system, and a method of producing such article by heating the article for a sufficient time, and at a sufficient temperature, in a dilute, hydrogen-containing atmosphere to so move the chromaticity point.

This is a division of application Ser. No. 08/585,056, filed Jan. 9,1996, now U.S. Pat. No. 5,639,701.

This application claims the benefit of U.S. Provisional Application No.60/005,014 filed Oct. 10, 1995, entitled PERMANENT TINT PHOTOCHROMICGLASSES, by David J. Kerko and Brent M. Wedding.

FIELD OF THE INVENTION

Photochromic glasses having a permanent tint imparted by colorants addedto the glass composition.

BACKGROUND OF THE INVENTION

Photochromic glasses containing, in their compositions, silver and oneor more halogens are well known. The term "halogens" is used here toinclude only chlorine, bromine and iodine. Such photochromic glasseswere first described in U.S. Pat. No. 3,208,860 (Armistead et al.). Asthere described, the glass compositions, in addition to silver andhalogens, consist essentially by weight on the oxide basis of 40-76%SiO₂, 4-30% B₂ O₃, 4-26% Al₂ O₃ and R₂ O selected from 2-8% Li₂ O, 4-15%Na₂ O, 6-20% K₂ O, 8-25% Rb₂ O and 10-30% Cs₂ O, these essentialsconstituting at least 85% of the glass composition.

A second generation of silver halide-containing, photochromic glasses isdescribed in U.S. Pat. No. 4,190,451 (Hares et al.). The compositions ofthese glasses are described as consisting essentially of 0-2.5% Li₂ O,0-9% Na₂ O, 0-17% K₂ O, 0-6% Cs₂ O, 8-20% total R₂ O, 14-23% B₂ O₃,5-25% Al₂ O₃, 0-25% P₂ O₅, 20-65% SiO₂, 0.004-0.02% CuO, 0.15-0.3% Ag,0.1-0.25% Cl and 0.1-0.2% Br, RO-free other than CuO, and the weightratio of Ag:(Cl+Br) is 0.65-0.95.

Photochromic glasses based on silver halide crystals are produced bydissolving silver and halogens in a suitable base glass melt. Upon acontrolled thermal treatment, a silver halide crystal phase isprecipitated in the glass. These crystals are subject to photolysis,that is, separation under the influence of short wave radiation such ascontained in sunlight, and recombination upon removal of the radiation.This gives rise to photochromism, that is, reversible darkeningbehavior. The glass is essentially colorless in the faded, orundarkened, state.

It is possible to impart permanent coloration to photochromic glasses byadding conventional glass colorants to the glass batch in minor amounts.Such known colorants include oxides of the transition metal oxides,nickel, cobalt, and chromium. This practice of imparting a permanenttint to photochromic glasses is described in the Hares et al. patentnoted above.

More recently, another method of inducing permanent coloration in aphotochromic glass has been described. This method involves thermaltreatment of the glass in a reducing atmosphere. This involves at leastpartially reducing the silver to impart the permanent coloration. Aflowing stream of hydrogen is commonly prescribed as the reducingatmosphere. The method is described in detail, for example, in U.S. Pat.No. 4,240,836 (Borrelli et al.).

Difficulties have been encountered in using glass colorants, such as thetransition metal oxides, to permanently tint otherwise transparentphotochromic glasses. In particular, in some glasses the cosmeticeffects obtained using such chromophores tend to have an undesirablegreenish cast.

The visible absorption characteristics imparted by the chromophores ofthese oxides are influenced by the glass matrix. Likewise, photochromicperformance depends on the glass matrix. Consequently, commonly knowntechniques for adjusting transmission characteristics are limited.

There are two recognized methods for decolorizing a greenish color suchas that induced by iron oxide in glass. One method involves adding anoxide of manganese. The other involves adding the oxide of the rareearth erbium. The addition of manganese is ruled out because, in amountsadequate for the purpose, it has an adverse effect on photochromicperformance. The cost, and the uncertain availability, of erbium oxideof adequate purity is such as to make that method undesirable.

It is a purpose of the present invention to provide a viable alternativeto these known methods. A further purpose is to provide an effectivemethod of neutralizing the greenish cast encountered in somephotochromic glasses permanently tinted by chromophores of transitionmetal oxides. Another purpose is to provide a means of shifting thechromaticity point of a permanently tinted, photochromic glass toneutralize the greenish cast of the glass color. A particular purpose isto provide photochromic sunglasses having a more pleasing appearance forthe wearer.

SUMMARY OF THE INVENTION

One aspect of the invention resides in a photochromic glass havingcolorant incorporated in the glass composition to impart a permanenttint, and having its chromaticity point shifted from the blue-green sidetoward the red side of the white point in the CIE chromaticitycoordinate system.

Another aspect of the invention resides in a method of producing suchphotochromic glass article by heating the article for a sufficient time,and at a sufficient temperature, in a dilute, hydrogen-containingatmosphere to move the chromaticity point from the blue-green sidetoward the red side of the white point in the CIE chromaticitycoordinate system.

PRIOR ART

In addition to patents already cited, patent literature deemed relevantis supplied separately.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 in the accompanying drawing is a graphical depiction of twocomparative glass transmittance curves.

FIG. 2 is a graphical illustration depicting the effect of time andtemperature on chromaticity coordinates in accordance with the presentinvention.

DESCRIPTION OF THE INVENTION

The present invention relates to development of a permanently colored,or tinted, photochromic glass article. As noted earlier, transitionmetal oxides, such as NiO, CoO and Cr₂ O₃, are well known as a means ofintroducing permanent chromophores, that is, color sources, into glassesincluding photochromic glasses. Such practice has tended to producecolors having an undesirable greenish shade or cast in certain preferredbase glasses. The present invention is based on our finding that suchtendency can be avoided.

We have found that the undesirable greenish shade is the result ofpositioning the chromaticity point to the left of the white point, thatis Illuminant C, on a chromaticity diagram representing glass color.Illuminant C is the white point in the 1931 CIE chromaticity coordinatesystem. It was used in all tristimulus value computations herein. Ourinvention provides a means of shifting the chromaticity point of a glasstoward the opposite, reddish side of the white point.

We have found that this desirable movement of the chromaticity point canbe achieved by thermally treating a photochromic, permanently tintedglass in a reducing atmosphere. Development of the desired shift inchromaticity point, however, requires closely controlling the thermalreduction treatment.

The controlled treatment that is characteristic of the present inventionis generally applicable to photochromic glasses having (1) an R₂ O--Al₂O₃ --B₂ O₃ --SiO₂ base glass composition, (2) a silver halide crystalphase developed in the glass to impart photochromic behavior, and (3) upto about 1% by weight of a metal oxide having a chromophore to supply apermanent color or tint to the glass. Suitable base glass compositionsare disclosed in the Hares et al. patent noted earlier. The subjectmatter of this patent is incorporated in its entirety by reference.

In accordance with color theory, opposing primary colors are red/greenand yellow/blue. Thus, to make a color less yellow, one adds some blue.In the present case, the objective is to reduce the greenish hue of aglass article, that is, to move toward the red.

With some photochromic glass compositions, e.g., those shown in FIG. 2of U.S. Pat. No. 4,240,836, it is not possible to obtain reddishcoloration by hydrogen firing without using an additional processingstep. U.S. Pat. Nos. 4,537,612 and 4,710,430 (Borrelli et al.) teach amethod of accomplishing this by a preliminary ion exchange. Thepreferred embodiment of the present invention does not require such anadditional processing step. However, glasses so treated are includedwithin the scope of the present invention.

Experience gained in developing our invention has shown that carefulcontrol of both time and temperature is required to exercise the desiredeffect on permanent coloration in a photochromic glass. In order toexercise such control, we have found it difficult to employ thecustomary practice of using a flowing stream of pure hydrogen, or aconcentrated forming gas, such as cracked ammonia, for reduction ofsilver halide crystals.

Accordingly, we use a dilute hydrogen atmosphere containing 5-15% H₂ inan inert gas, preferably nitrogen. We prefer a H₂ --N₂ mixture in aratio of about 1:10 by volume. The thermal reduction process is thusslowed sufficiently to allow any time-temperature fluctuations toaverage out. This tends to reduce variability of color produced overtime in a production line. It is particularly important in an articlesuch as sunglasses where variation in lens color is most undesirable.

In addition to atmosphere control, both time and temperature arecontrolled. In general, temperature is controlled within the range of380°-420° C., preferably 390°-410° C., for a time of 15-45 minutes.Firing for about 20 minutes at about 400° C. permits good control.

Our prior patents teach that the hue of coloration produced by hydrogentreatment is strongly temperature dependent. The amount of colorimparted depends on treatment time and hydrogen concentration in thetreating atmosphere. Practice of the present invention requires controlof all three variables. Treatment at temperatures above about 420° C.,and/or use of an atmosphere having a high concentration of hydrogen,does not permit proper color control, particularly on a production line.

The invention is further described with reference to three commercialphotochromic glasses known under Codes 8114, 8115 and 8135. Theseglasses, contain amounts of cobalt and nickel oxides within a range of0.05-0.25% to impart permanent coloration. The glasses, respectively,exhibit light gray, light brown and dark gray colors. Theircompositions, in approximate weight percent as calculated from the glassbatch, are as follows:

                  TABLE I                                                         ______________________________________                                        Component  8114         8115    8135                                          ______________________________________                                        SiO.sub.2  56.46        56.46   56.4                                          Al.sub.2 O.sub.3                                                                         6.19         6.19    6.2                                           B.sub.2 O.sub.3                                                                          18.15        18.15   18.1                                          Li.sub.2 O 1.81         1.81    1.8                                           Na.sub.2 O 4.12         4.12    4.1                                           K.sub.2 O  5.72         5.72    5.7                                           TiO.sub.2  2.07         2.07    1.9                                           ZrO.sub.2  4.99         4.99    5.0                                           Ag         0.254        0.254   0.24                                          CuO        0.0068       0.0068  0.0057                                        Cl         0.210        0.210   0.215                                         Br         0.169        0.169   0.16                                          CoO        0.022        0.0138  0.082                                         NiO        0.030        0.128   0.144                                         ______________________________________                                    

Lenses produced from each of the glasses tend to have a greenish cast.Thus, Code 8114 lenses have a slightly blue-green cast; Code 8135 lenseshave a more pronounced blue-green cast; a Code 8115 lens, a khaki tint.A brief treatment of a polished sample of each glass in a dilutehydrogen gas atmosphere at a temperature near 400° C. removes thegreenish appearance present in the untreated glass.

A polished Code 8135 glass plano-power lens having 2 mm-thickness wasplaced in a tube furnace and heated to 390° C. in a flowing nitrogenatmosphere. After the temperature had stabilized, pure hydrogen wasadded to the nitrogen stream such that the flow rates were about onepart hydrogen to ten parts nitrogen. Twenty minutes later the hydrogenflow was shut off and the mixture was purged from the furnace tube bycontinuing the nitrogen flow.

After cooling, the visible wavelength spectral transmittance of the lenswas measured. From those data, tristimulus values were found by aweighted ordinate method using the 1931 CIE standard observer andIlluminant C. The chromaticity coordinates found from the tristimulusvalues are given in the table, as are the luminous transmittances (Y).Data for Illuminant C are also given.

    ______________________________________                                                Before         After  C                                               ______________________________________                                        x       .3023          .3164  .3101                                           y       .3158          .3160  .3161                                           Y       47.95          39.60  100.0                                           ______________________________________                                    

Even though the colors before and after treatment are both very nearneutral, one observes substantial changes in visible appearance. This isa consequence of shifting the chromaticity point from the blue-greenside to the red side of the white point.

FIG. 1 is a graphical depiction of spectral transmittance. Wavelengthsare plotted in millimicrons (mμ) on the horizontal axis. Transmittanceis plotted in percent on the vertical axis. FIG. 1 displays plots of thespectral transmittance of the Code 8135 lens before and after tinting.Curve A represents the transmittance of the untreated glass, that is,prior to the thermal reduction. Curve B represents the transmittance ofthe glass sample after treatment as described above. It may be notedthat the relatively greater spectral transmittance in the wavelengthinterval 460-550 nm imparts the slight blue-green coloration to the lensprior to treatment as shown by Curve A. This effect has been reduced bythe treatment in accordance with the invention as shown by Curve B.

FIG. 2 is a graphical representation depicting the effect of time andtemperature variations on chromaticity coordinates. Δx representschanges in a selected nominal x coordinate, and is plotted on thehorizontal axis. Δy represents changes in a selected nominal ycoordinate, and is plotted on the vertical axis. FIG. 2 shows achromaticity target box ABCD tentatively selected as an aid indeveloping the invention.

Box ABCD is centered on a pair of x and y coordinates selected as atarget value and designated by N. It represents permissible variationsfrom that target value. Δ values are plotted to more vividly depictchanges that may occur. The vectors, shown as arrows, illustrate thepositive and negative coordinate variations from a fixed point, such asN, that are effected by variations in time and temperature. The vectorsindicate that, given a targeted time and temperature, it is necessary tomaintain the time at ±10% of that value, and to maintain the temperatureat ±3° C. of the temperature value. These are the maximum variationspermissible to stay within the targeted chromaticity target box in thisexample.

We claim:
 1. A method of altering the visible absorption characteristicsimparted by metal oxide glass colorants in a permanently tintedphotochromic glass article, the article comprising an R₂ O--B₂ O₃ --Al₂O₃ --SiO₂ base glass, silver halide crystals precipitated throughout theglass, and one or more metal oxide glass colorants selected from thegroup consisting of NiO, CoO, and Cr₂ O₃ in the base glass composition,which comprises thermally treating the article in a dilutehydrogen-containing atmosphere for a sufficient time and at a sufficienttemperature to move the chromaticity point of the glass from theblue-green side toward the red side of the white point in the CIEchromaticity coordinate system.
 2. A method in accordance with claim 1which comprises thermally treating the article in an atmospherecontaining an inert gas and 5-15% hydrogen.
 3. A method in accordancewith claim 1 which comprises thermally treating the article in atemperature range of 380°-420° C.
 4. A method in accordance with claim 1which comprises thermally treating the article for a time of about 15-45minutes.
 5. A method in accordance with claim 1 which comprisesthermally treating the glass article in a reducing atmosphere composedof about one part H₂ to 10 parts N₂ for about 20±2 minutes at atemperature of about 400°±3° C.
 6. A method of neutralizing the greenishcast in a photochromic glass containing silver halide crystals andpermanently tinted with a glass colorant metal oxide selected from thegroup consisting of NiO, CoO, and Cr₂ O₃ which comprises thermallytreating the article in a dilute hydrogen-containing atmosphere for asufficient time and at a sufficient temperature to move the chromaticitypoint of the glass from the blue-green side toward the red side of thewhite point in the CIE chromaticity coordinate system.